The present invention relates to a new and improved measuring arrangement for an apparatus for electrostatic coating of grounded objects for the purpose of measuring the ground or earthing resistance.
In all those techniques where objects or articles are electrostatically coated it is of particular importance that the objects are properly grounded so that there is maintained the potential difference between the electrostatically charged particles of the coating material and the objects. Furthermore, grounding of the objects is undertaken to prevent such from becoming charged by the coating operation itself and thus impairing the safety of the operating personnel.
In conjunction with equipment for electrostatic coating the ground resistance was previously determined by periodically carrying out direct resistance measurement operations. In other words by contacting the object there was impressed thereon a certain potential and the current flowing through the object to ground was measured. In this way it was possible to easily determine the ground or earthing resistance. As simple as this measuring technique appears to be it has been found to possess considerable drawbacks when employed in connection with automatic coating installations where the objects are conveyed past the coating nozzle. Firstly, the objects moved by means of a conveying device past the coating apparatus are themselves grounded via the conveying device. This conveying device itself necessarily possesses mechanically moved components and it is so-to-speak unavoidable that with time there are formed at such components scale and lubricant layers. As a result, at least when working with low voltages, there is influenced the quality of the electrical contact between the objects transported by the conveyor and ground. The heretofore employed conventional techniques in such instances not only occasionally delivered values of the ground resistance but such values were unreliable if, owing to contact of the object, there was impressed thereon a relatively low voltage which was not capable of penetrating through the insulating scale- and lubricating film.
The quality of the electrostatic coating is however also dependent upon the value of the ground resistance at high voltages at the region of the coating apparatus. If a direct ground resistance measurement is undertaken at the region of the coating apparatus then, as already mentioned, the object must be contacted in order to apply thereat a relatively low voltage corresponding to the ohmic measurement circuit. Apart from the fact that when working with this technique it is not possible to determine any reliable value for the ground resistance it is further to be noted that at the point of contact it is not possible to carry out any, or at the very best, a less compact coating, and therefore there is required a subsequent touching-up of the blank or else locations will appear which have been insufficiently coated.